1. Field of the Invention
The present invention relates to a microstrip antenna for mobile communication use, the antenna comprising a dielectric substrate, a patch, and a ground conductor plate, the patch and the ground conductor plate being disposed on one surface and the other surface of the dielectric substrate.
2. Description of the Related Art
In mobile satellite communication systems, communications are made between a mobile station and a base station and between mobile stations. An antenna for such systems should be small and light weight. In addition, the antenna is required to transmit and receive circularly polarized radio waves with different frequencies. Moreover, to secure a predetermined communication quality level, the transmission channel should have output power of several watts or more. In this condition, if the loss of a transmission feed circuit is large, the output power of a power amplifier should be increased. Thus, the size of the power amplifier becomes large. In addition, a decrease of the efficiency of the power amplifier results in heat generation. Thus, the size of the heat sink for the power amplifier becomes large.
When the transmission output becomes large, a device for separating a reception channel from a transmission channel is required so as to prevent a transmission signal from leaking out to a reception signal. As a separating device for use with an antenna which is common to transmission and reception, a diplexer is generally used. On the other hand, for an antenna which is not common to transmission and reception, a filter is used. In particular, for an active array antenna, each antenna element requires one separating device for separating reception from transmission. The size and weight of these separating devices such as diplexers and filters are larger and heavier than those of the antenna elements. As the number of antenna elements increases, the weight and volume of the entire antenna increases. Thus, the spatially occupied region of the antenna becomes large. This large and heavy antenna is not suitable for the antenna of a mobile station. One technique for reducing the size of the antenna is to get the isolation between reception and transmission by the cooperation of the antenna elements to reduce the demand for the filters and the diplexers.
FIG. 25 shows a construction of a microstrip antenna proposed by Shiokawa et al., Microstrip Array for Aeronautical Satellite Communications, IEICE of Japan, Technical Report, A.P86-60.
This antenna is a circularly polarized wave antenna with separate elements for transmission and reception. This antenna uses a frequency selectivity between a transmission patch 100 and a reception patch 101. The isolation between the transmission element and the reception element of this antenna is approximately -28 dB. Since the required isolation is in the range from -60 to -70 dB, a band pass filter should be used to obtain the required isolation level. Moreover, according to this antenna, the transmission patch 100 is superimposed on the reception patch 101, and the area of the antenna is small. However, such structure leads to a complicated construction of the antenna. In addition, since coaxial cables 103, 104, 105, and 106 are used, they should be soldered. Furthermore, to separate the transmission patch 100 from the reception patch 101, the reception patch 101 should be formed in a ring shape. Thus, a short conductor plate 107 should be shortcircuited to the reception patch 101 with a large number of short pins 108. Therefore, the construction of the antenna is complicated, thereby increasing the number of the production steps and raising the production cost. In addition, to generate a circularly polarized wave, a 90.degree. hybrid for generating a phase difference of 90.degree. should be provided between the coaxial cables 105 and 106.
FIG. 26 is a plan view showing a construction of a conventional microstrip antenna having four antenna elements for both transmission and reception. Signals are fed with feed lines on the same plane. The antenna generates circularly polarized waves. This antenna has been disclosed in Japanese Patent Laid-open Publication Serial No. HEI 2-116202.
As shown in the figure, according to this microstrip antenna, a microstrip line 141 arranged on the same surface of the rectangular patch 140a feeds a signal directly to an edge of the rectangular patch 140a, thereby generating a horizontally polarized wave with a frequency f.sub.1. On the other hand, a microstrip line 142 feeds a signal directly to the rectangular patch 140a, thereby generating a vertically polarized wave with a frequency f.sub.2. This antenna is provided with four rectangular patches 140a, 140b, 140c, and 140d as antenna elements. These rectangular patches 140a, 140b, 140c, and 140d are disposed in such a way that angles therebetween are 90.degree. on the same plane. In addition, two signals with frequencies f.sub.1 and f.sub.2 and a phase difference of 90.degree. are fed to each rectangular patch, thereby generating circularly polarized waves. However, the input impedance at the edge of the rectangular patch 140a is in the range from 200 to 300 .OMEGA., whereas the characteristic impedance of the feed line is 50 .OMEGA.. Thus, to match these impedances, transformers having a line length of .lambda. g/4 should be provided for both transmission and reception. Moreover, since this antenna is an array antenna, these transformers should be provided for each antenna element. Further, to perform beam scanning with a wide angle, the length of the interval between elements of the array antenna should be about a half wavelength of the signal. Thus, in a limited space, a feed line including an impedance transformer with a line length of .lambda. g/4 should be provided for both transmission and reception. Therefore, since the feed lines come close each other or to the antenna elements, a mutual coupling occurs. Thus, the condition where signals with the same amplitude and a phase difference of 90.degree. should be fed cannot be satisfied. Therefore, a circularly polarized wave cannot be properly generated. In addition, since a mutual coupling occurs between a transmission feed line or a transmission antenna and a reception feed line, the isolation between the transmission band and the reception band is deteriorated. As reported by AP-S90 pp 803-806, SELF DIPLEXING CIRCULARLY POLARIZED ANTENNA, according to this antenna, the isolation between the transmission band and reception band is at most in the range from -20 to -23 dB. Moreover, when the thickness of the substrate is increased for widening the band of the antenna, due to high order mode TM.sub.20 a mutual coupling occurs between the transmission port and the reception port, thereby deteriorating the isolation between the reception and transmission.